My invention relates to automatic control valves operated either by a modulating electric or pneumatic signal to control very minute flow rates, as is customary in the operation of pilot plants in the chemical or petroleum industry. Control valves presently in use for this purpose are either of the low-lift variety utilizing a needle-type plug or the long lift cylindrical plug type with precision machined longitudinal scratch, commonly referred to as micro-splined plugs. Both of these types exhibit some problems. The needle-type plug is commonly limited to a tapered angle of 30.degree. to avoid self-locking against the seat ring bore. This in turn reduces the stroke whenever small orifice sizes are required. For example, the maximum usable stroke for control with a 1/32 inch orifice is only 0.055 inch, i.e., much too short to achieve any meaningful positioning accuracy with conventional actuating devices. Another drawback is the very high tendency for fluids to cavitate with needle plugs resulting in early destruction of the valve parts.
The micro-splined plugs require extremely accurate machining of splined grooves having depths of less than 0.001 inch. The minimum controllable area, limiting the lowest flow rate that can be regulated, is given by the radial clearance between the external diameter of the plug and the orifice bore. With a normal radial clearance of 0.00025 inch, the minimum controllable area of such a plug with 1/4 inch diameter is 2.times.10.sup.-4 in.sup.2 or 25% of the maximum area of 1/32 inch dia. orifice, thereby limiting the ratio of max. to min. control area or rangeability to less than 4:1.
My invention, on the other hand, reduces the amount of radial clearance around the valve plug to virtually zero by the use of a spring-loaded and deformable plastic seat ring that is squeezed around the plug in the closed or nearly closed valve position. This is a major improvement over my prior invention shown in U.S. Pat. No. 3,997,141, which had no soft seat and, thereby, no tight shutoff capability.
The latter feature is important when extremely small flow rates of gaseous media need to be controlled. For example, the leak rate of 1/4" diameter needle trim with metal-to-metal may well exceed 10 cc/min at 100 psi pressure drop of which is well within the control range of many small flow valve applications.
To obtain tight shutoff of valves handling high pressure of 1000 psi or more has always been a challenge to a valve designer. One successful way of doing this is illustrated in my prior U.S. Pat. No. 3,809,362. Here, following metal-to-metal contact of a plug with a moveable seal, a plastic such as PTFE is compressed at a very high rate of pressure so that the plastic begins to flow, thereby, providing a highly viscous sealing media between the fluid pressure and the valve outlet. While this solution is very effective, it cannot be scaled down to plug sizes smaller than about 1/2" diameter.
A solution to find a workable alternative to such a trim mechanism is given in my present invention where a variable spring load is able to highly compress an annular PTFE (or similar plastic) insert as explained in more detail in the following description.
To have a spring opposed valve plug in itself is not a new art. For example, publication 8015-3 of Samson AG of West Germany does show such a plug where the spring serves to retract a needle plug from a separate seat ring which is retained within a housing by a seat nut. This poses various serious disadvantages: First, serious alignment problems can occur whenever the seat ring bore is not machined together with the plug guide realizing that the clearance between plug and seat bore may be as low as 0.0002"; secondly, any contact between the seat ring and the housing can cause an additional leakage path for the process fluid.
My invention overcomes these limitations in as much as my plug guide diameter and seat bore are machined simultaneously and within the same part. This also eliminates the potential leak path mentioned above.
Finally, by choosing a spring with a variable spring rate, I can limit the spring force to a moderate load during normal valve travel and to generate a high load (for deformation of the PTFE ring) near the point of valve closure. This reduces friction on the valve plug during normal operation.
Another way to obtain high rangeability or a low minimal controllable flow coefficient is to have a long frictional flow path between plug diameter and seat ring bore. This means a relatively long plug travel exercised by a pneumatic piston actuator who's precision in positioning the valve plug is enhanced by an attached valve positioner. Both actuator and positioner should be as compact as possible. To this end, I have devised a way to conduct air from the positioner to the pressurized cylinder of the actuator without need for an external air tubing, and I have, furthermore, found a way to adjust the initial travel position or zero setting of the actuator without resorting to penetrating levers or adjustment screws prone to leak and to add significant cost to the valve.